Diesel engine for a large ship mainly uses heavy oil as a fuel, and thus, an exhaust gas discharged from a combustion chamber of the engine contains a large amount of highly corrosive sulfide and the like. For this reason, in an exhaust valve is used a metal material that is highly resistant to high temperature corrosion called S attack or V attack, which is caused by the contact with such an exhaust gas flow. As examples of a material having excellent high temperature corrosion resistance, known are an Ni—Cr—Al system Ni-base alloy such as Nimonic 80A and Inconel 718, and a Co-base alloy such as Stellite (“Nimonic”, “Inconel” and “Stellite” are registered trademarks).
An exhaust valve of a diesel engine has a shaft part and an umbrella part (disc part) including a fire contact surface and a seat surface. The umbrella part is required to have high toughness such as corrosion resistance and abrasion resistance in high temperature environment. On the other hand, it is considered preferable that the shaft part has a certain degree of machinability for incorporating in an engine, that is, has a toughness not increased so much. For this reason, there has been proposed a hybrid-type engine valve using such a high corrosion resistance alloy in only an umbrella part. On the other hand, considering easiness of manufacturing, an integrated exhaust valve having a shaft part and an umbrella part that are integrated with each other is advantageous, and there has been also proposed a gradient material type integrated exhaust valve in which mechanical properties have been adjusted in each of its shaft part and umbrella part.
For example, Patent Document 1 discloses an integrated exhaust valve, as an exhaust valve for a diesel engine, in which its mechanical strength has been partially increased by applying cold working to a face surface of an umbrella part by using an Ni—Cr—Al system alloy which contains Cr in an amount larger than that of Nimonic 80A containing about 20% of Cr and has excellent high temperature corrosion resistance. In detail, an outline of the exhaust valve is obtained by using an Ni-base alloy having a component composition of, in % by weight, C: ≤0.1%, Si: ≤1.0%, Mn: ≤1.0%, Cr: more than 25 to 32%, Ti: more than 2.0 to 3.0%, Al: 1.0 to 2.0% and Co: 12 to 20%, and cold working is then applied to a face surface of its umbrella part, thereby partially increasing mechanical strength of the face surface.
Patent Document 2 discloses an integrated exhaust valve in which mechanical strength of a site requiring mechanical strength has been partially increased by build-up welding, as an exhaust valve for a middle or high speed type diesel engine used in a small ship or a power generator. In Patent Document 2, an umbrella part is formed by die forging by using a precipitation-hardened Ni—Cr—Al system alloy to obtain an outline of an exhaust valve including a shaft part, and a first heat treatment is applied to the exhaust valve until exceeding a peak of mechanical strength (mainly hardness) and softening, that is, until reaching so-called overaging. A face surface of the umbrella part is subjected to grooving, build-up welding is performed thereon, and a second heat treatment is then performed. As a result, the shaft part has been overaged, and thus, hardness thereof is reduced than a peak value, and additionally machinability is improved. This facilitates cutting performed according to the need such as an engine mounting process. On the other hand, a build-up welded part on the face surface can be improved in corrosion resistance at high temperature by the second heat treatment. As a result, sealing properties can be enhanced.    Patent Document 1: JP-A-2000-328163    Patent Document 2: JP-A-2014-169631